1. Field of the Invention
This invention relates generally to wheel trim and more particularly to improvements in wheel cover retention systems.
2. Antecedents of the Invention
In the various systems for mounting wheel covers to vehicle wheels, the retentive forces between the wheel cover and wheel were applied either in radial or axial directions.
In some radial retention systems, as exemplified by U.S. Pat. Nos. 4,232,907 and 4,247,151, a plurality of sharp metal teeth or barbs on the ends of a spring leaf, engaged a wheel rim. The spring leaves were individually mounted to the wheel cover or comprised part of a metal grip ring which was received within an annular channel on the inner face of a wheel cover, as shown in U.S. Pat. No. 3,876,257, owned by the assignee of the present invention.
Radial retention systems suffered from certain disadvantages, among which were that the sharp barbs were prone to scratch paint from the surface of the wheel rim, exposing the rim to moisture and road salts, which resulted in accelerated corrosion.
Also, stresses were generated by the engagement between the spring leaves and the wheel rim. In conjunction with plastic wheel covers, these stresses often resulted in cracks or other failures. Further, due to the heat generated by the vehicle braking system and tire flex, plastic wheel covers often attained elevated temperatures, which degraded their structural rigidity. This resulted not only in distortion of plastic wheel covers, but also reduced grip and consequent loss of wheel covers as evidenced by the numerous wheel covers which have accumulated on the sides of our streets and highways.
In an attempt to avoid scratching of paint from wheel rims and the resultant corrosion, radial retention systems have also employed non-scratch leaf springs, as exemplified in U.S. Pat. No. 5,542,751, assigned to the assignee of the present invention. While non-scratch leaf springs reduced corrosion, they did not resist rotation of the wheel cover relative to the wheel. When such rotation occurred, contact between a valve stem and the wheel cover surfaces which defined a valve stem cut out was possible.
Axial retention systems have been exemplified by attempts to utilize vehicle lug nuts for mounting wheel covers as illustrated in U.S. Pat. No. 2,031,092 and U.S. Pat. No. 4,787,681. The placement of wheel cover components between the abutting surfaces of the lug nut and the wheel altered the characteristics of the wheel mounting arrangement itself. Further such retention systems often required wheel cover assembly steps as an aspect of the mounting procedure. The lug nuts were removed, a mounting portion of the wheel cover was placed against the wheel, the lug nuts were then reapplied and tightened against the mounting portion of the wheel cover. Thereafter, further portions of the wheel cover were attached to the mounting portion of the wheel cover.
Attempts have also been made to mount wheel covers by engaging threaded caps over portions of threaded lugs which projected beyond the ends of lug nuts, as illustrated in U.S. Pat. No. 4,895,415 and U.S. Pat. No. 5,222,785. The cap was tightened down against the end of the lug nut, compressing the wheel cover between the lug nut and the cap. The periphery of the wheel cover engaged the peripheral edge of the wheel rim. This engagement resulted in axial outward flexing of the periphery of the wheel cover. The engagement between the periphery of the wheel cover and the periphery of the wheel rim resulted in rubbing contact and the generation of noises, especially during the occurrence of wheel rim flex.
A further axial retention system is illustrated in U.S. Pat. No. 4,998,780 wherein a cap engaged external threads on a lug nut. While a fixed axial cap stop was not present in this system, contact between the wheel cover and the periphery of the wheel rim established a yieldable resistance for cap tightening. Axial tolerances in wheels, however, permitted axial variances in the order of 8 mm between the peripheral edge of the wheel rim and the lug apertures of a center disc of the wheel. This resulted in wide range of stress levels to which the wheel cover was subject.
Axial retention systems premised upon engagement of a wheel cover and a radial flange of a lug nut have also been proposed, as illustrated in U.S. Pat. Nos. 5,520,445, 5,297,854 and U.S. Pat. No. 4,842,339. These systems generated similar stress resulting from the engagement of the periphery of the wheel cover with the peripheral edge of the wheel rim, with the stress levels varying widely, due to the tolerances permitted in the axial distance between the peripheral edge of the wheel rim and the lug apertures.
In U.S. Pat. No. 3,356,421, a helical coil spring biased axial retentive force was employed, however, the engagement between the peripheral edge of the wheel cover and the wheel weight channel of the wheel rim provided the limit stop and the range of axial stress exerted on the wheel cover also varied with axial wheel tolerances.
Generally, the axial stresses exerted upon a wheel cover in conjunction with axial retention systems comprised the axially inward retentive force, applied by the cap or lug nut engagement along a lug circle surrounding the center of the wheel cover and the counterbalancing axially outward forces applied about the periphery of the wheel cover by engagement with the wheel rim.
Such stresses placed limitations upon the range of materials from which plastic wheel covers could be fabricated and generally required relatively stiff high grade plastics such as ABS+PC (acrylonitrile--butadiene--styrene/polycarbonate blend) with integral internal web reinforcement and a thickness in the order of approximately 3 mm. Lightweight and/or lower grade plastics or thinner wheel covers were not well suited to withstand the stresses and elevated temperatures encountered. Further, thin walled metal wheel covers were particularly unsuited for axial retention systems of the type disclosed in U.S. Pat. No. 4,998,780. The axially inward retentive forces generated a concave depression on the face of the wheel cover.